In electrophotographic imaging processes, such as in electrophotographic coping machines employing reusable photoconductors, an electrostatic latent-image charge pattern is formed of a photoconductive element which can be in the form of a belt, drum or plate. By treating the charge pattern with a dry developer containing charged toner particles, the latent image is developed. The toner pattern is then transferred to a receiver such as a sheet of paper to which it is fixed by fusion or other means.
A problem in transferring the developed image in this kind of process is that the attraction of the toner to the electrophotographic element can cause incomplete transfer to the receiver element. The resulting transferred image on the receiver has hollow characters and other defects. The problem is especially severe when the image is transferred by pressing a receiver element such as a paper sheet into contact with the toned surface of the photoconductive element..
Efforts to solve the image-transfer problem have included providing abhesive or release coatings to photoconductive elements. A drawback of this attempt to solve the problem is that an insulating, nonphotoconductive overcoat can interfere with the photoconductive properties of the element. If the coating is thick, it can materially reduce the electrophotographic speed or sensitivity. Even if thin, an insulating overcoat layer can shorten the life of a photoconductor such that it cannot be regenerated for repeated use. This is believed to be caused by the trapping of residual charges between the insulating coating and the photoconductive layer. If the surface is merely coated with a soft release substance such as a metal stearate, the coating rapidly wears off and the transfer problem reappears.
Although evidently not providing improvded image transfer, a tetrafluoroethylene telomer identified by the trademark "Vydax 1000" of E. I. duPont deNemours and Company is disclosed as a component of an overcoating composition for an electrostatographic photoreceptor in the patent to McMullen, U.S. Pat. No. 4,012,255. It is incorporated with a crosslinkable polymer coated over a selenium arsenic photoconductor. An insulating overcoating such as this would have the disadvantages previously mentioned of reducing the electrophotographic sensitivity and of interfering with the regenerative capability of the element. Furthermore, the crosslinked polymer, being abrasion-resistant, would not continually yield a fresh film of telomer as the surface is rubbed at a cleaning station or otherwise. Hence, the improved image transfer obtained in accordance with the present invention would not be expected.
In accordance with the present invention, a novel reusable electrophotographic element is provided from which, even after many cycles of use, toner can readily be uniformly transferred to a contiguous receiver element by pressure rollers or other means with minimal image defects.